Producing methods for fine particles are roughly divided into two kinds of a bottom-up method for producing fine particles by nuclear growth and a top-down method for producing fine particles by grinding a large lump; the top-down method has been mostly used from the viewpoint of low costs and excellent high-volume production capability. One of methods for obtaining fine particles by the top-down method is a medium grinding method for grinding by using a medium. For example, in a ball mill using a ceramic ball as a medium, a grinding ball such as zirconia, a product to be ground, solvent, and optionally a dispersant are added into a vessel to move the grinding ball by revolving the vessel and then grind by shear force caused between the grinding balls (at a point of application). Accordingly, the larger the particle diameter of the grinding ball becomes and the heavier the weight of the ball becomes, the greater the shear force caused at the point of application becomes, so that sufficient grinding may be performed at the point of application. On the other hand, the smaller the particle diameter of the grinding ball becomes, the more the number of the points of application increases, so that grinding may be efficiently performed. However, there has been a problem that the larger particle diameter of the grinding ball for increasing the weight of the grinding ball decreases the number of the points of application, while the smaller particle diameter of the grinding ball for increasing the number of the points of application decreases the shear force at the point of application. Thus, the use of the grinding ball made of a substance as high in density as possible has made compatible the maintenance of the shear force and the increase of the number of the points of application. However, it has been difficult that a substance having an amorphous structure, such as glass particles, is ground to obtain particles having a particle diameter of 0.7 μm or less.
Then, the use of the grinding ball composed of plural kinds of particle diameters as a grinding medium has been proposed as a grinding method for efficiently obtaining fine particles; specifically, in a horizontal cylindrical mixing method, grinding is performed by complexly using two kinds or more of balls having different sizes of 10 to 100 times and 100 to 1000 times with respect to the diameter of particles to be ground to obtain micron-level powder particles (refer to Patent Document 1). However, the above-mentioned method did not allow particle diameter on the order of submicrons to be obtained.
Patent Document 1: Japanese Unexamined Patent Publication No. 9-253517 (Claim 1 and so on)